Studies on the mechanism of action of M. Luteus polynucleotide phosphorylase have shown that "de novo" synthesis catalyzed by the enzyme yields long polymers of defined chain length. The homogenous Ca2 ion-dependent cyclic nucleotide phosphodiesterase activated by the Ca2 ion-dependent regulator (CDR), was shown to be the "high Km" enzyme which hydrolyzes both cAMP and cGMP. The enzyme has a molecular weight of 135,000 plus or minus 10,000, its catalytic subunit has a molecular weight of 59,000. Using chemically modified CDR, we were able to show that the catalytic subunit binds to the regulatory protein. It was also shown that the enzyme interacts physically with the inhibitory protein previously described in this laboratory. Other proteins under CDR control, smooth muscle and non-muscle cell myosin light chain kinases were purified by affinity chromatography on CDR coupled to Sepharose. Their regulation by CDR is presently under study.